Conventionally, there are image forming devices, such as electrographic printers, that include an exposure part configured from an array of a large number of light emitting elements. Light emitting diodes (LEDs), organic electroluminescent (EL) elements, light emitting thyristors and the like are used as the light emitting elements.
In such image forming devices using the light emitting thyristors, a driving circuit and the light emitting thyristor are provided at a ratio of 1:1 or 1:N (N>1). By passing electric current between an anode and a cathode of the LED, the light emitting state and non-light emitting state are switched.
The optical output of an LED in the light emitting state is determined by a value of the driving current. By adjusting the current, the amount of exposure energy to the exposure part is adjusted.
In addition, a configuration is known for the driving circuit, which has constant current characteristics by operating metal-oxide semiconductor (MOS) transistors in a saturation range and which drives the LEDs with a constant current.
The optical print head using the light emitting thyristors includes an anode driving circuit and a gate driving circuit inside the above-described driving circuit.
Japanese Laid-Open Patent Application Publication No. H09-109459 discloses N-gate light emitting thyristors formed by layering P and N-type semiconductors in a PNPN configuration. The light emitting thyristors include a P-type layer, which is the first, topmost layer, as an anode terminal, an N-type layer, which is the second layer, as a gate terminal, and an N-type layer, which is the fourth layer, as a cathode terminal. An optical print head driver integrated circuit (IC) for driving the light emitting thyristors includes an anode driving circuit and a gate driving circuit for driving the light emitting thyristors. A plurality of adjacent light emitting thyristors is grouped together. Anode terminals of the grouped thyristors are connected to each other. Gate terminals corresponding in different groups are connected to each other. The light emitting thyristors are driven by time division.
In the driving of the light emitting thyristors by time division, the gate terminal of a light emitting thyristor to emit light is at the low level (L level), and the gate terminal of a light emitting thyristor not to emit light is at the high level (H level). The driver IC for driving the light emitting thyristors is fabricated by using a complementary MOS (CMOS) process, which power source voltage is 5 V. In the gate driving circuit having a conventional configuration, the H level voltage is 5 V, which is approximately equivalent to the power source potential. The withstand voltage of the light emitting thyristors is only approximately 7 V, which is not enough for the power source voltage. As a result, the light emitting thyristor may be damaged by application of the H level voltage or may be degraded by application of the H level voltage for a long period of time.
This phenomenon is explained below. For example, the voltage is applied in a reverse direction between the N-type layer, which is the second layer, and the P-type layer, which is the third layer, of the light emitting thyristor when the thyristor does not emit light. The reverse breakdown voltage at the PN junction is known to be approximately 15 V.
The withstand voltage between the gate and cathode of the light emitting thyristor is determined based on the breakdown voltage. Considering an NPN bipolar transistor formed equivalently from an N-type layer, a P-type layer and an N-type layer, which are the second, third and fourth layers in a light emitting thyristor with a PNPN configuration, the withstand voltage between the gate and cathode of the light emitting thyristor is equivalent to the withstand voltage Vceo (max) between the collector and emitter of the NPN bipolar transistor with the open base terminal and is known to be given by the following equation according to “Basic of Semiconductor Devices” by A. S. Grove (translated by Tarui et al.), published by Ohmsha, Ltd., pp. 256-260:Vceo(max)=BV/(β)1/n where BV is the reverse breakdown voltage of the PN junction, β is a current gain of the NPN bipolar transistor, and n is a constant determined by experiments, which is n=3 to 6.
As an example, with an assumption that an experimental value n is 6 (n=6) with a GaAs material, that the current gain is 50 (β=50), and that the reverse breakdown voltage BV at the PN junction is 15 V (BV=15), the withstand voltage Vceo (max) is calculated as follows:Vcep(max)=15/(50)1/6=7.8 V
This value is not considered to be large enough for the withstand voltage in consideration of the operation at 5 V, which is the normal power source voltage for the driving circuit. This value is not preferable because it may cause problems, such as damaging the element by breakdown voltage, degradation of the light emitting thyristor by a long continuous application of voltage at 5 V (e.g., fluctuation of light emission amount and decrease of switching speed due to the lower current gain).
As apparent from the above exemplary calculation, in order to increase the switching speed of the light emitting thyristor, it is necessary to increase the current gain of the bipolar transistors. To do so, the base width of the NPN transistors must be reduced, and the thickness of the third layer (P-type layer) in the light emitting thyristor with the PNPN configuration needs to be thin.
However, although the current gain of the NPN transistors can be increased, the withstand voltage Vceo (max) between the gate and cathode of the light emitting thyristor decreases as shown in the above-discussed equation. In contrast, if the thickness of the third layer (P-type layer) is increased in the light emitting thyristor with the PNPN configuration in order to increase the withstand voltage between the gate and cathode of the light emitting thyristor, the base width of the NPN transistor is increased. This is not preferable because the current gain β is reduced and because the switching speed decreases.
As discussed above, the switching speed and the withstand voltage are contradictory from each other, and the withstand voltage cannot be simply increased. Therefore, solutions to these problems have been desired. In addition, similar problems occur when elements other than the light emitting thyristors, such as 3-terminal switching elements, are used.